The 3rd-Generation Partnership Project (3GPP) is currently developing specifications for a so-called fifth-generation (5G) cellular wireless communications system. A category of usage scenarios referred to by 3GPP as “Ultra-Reliable and Low Latency Communications” or “URLLC” is a family of usage scenarios identified by 3GPP to be addressed by future 5G cellular systems. A 3GPP document numbered 3GPP TR 22.862, v14.1.0, and titled “Feasibility Study on New Services and Markets Technology Enablers for Critical Communications,” defines URRLC as comprising use cases that are characterized by higher reliability, higher availability, and lower latency requirements, compared to conventional services.
Example of such use cases could include:                Remote control of machinery by a human operator located at a different place. This could, for instance, be used in the forest, mining, transportation industry, such that the use of remote control enables placing human operators at a safe, cost-efficient place that is different from the remote machine. In this scenario, the operator requires a highly reliable communication link to the controlled machine, which may carry control information, audio, video, haptic feedback, etc.        Intelligent traffic systems, to improve traffic safety and efficiency. This scenario can include cars, buses, and trucks that communicate with each other as well as with surrounding roadside equipment, pedestrians, etc. One example of such a use case is truck platooning, where trucks can lower fuel consumption by traveling very closely to each other. This requires a highly reliable and low-latency communication channel between the trucks, e.g., to coordinate braking.        Industrial factory automation, making it possible to improve factory efficiencies by connecting all sensors and actuators to the same network, enabling centralized control. Wireless technologies can play a large part in this, since wireless communications minimizes the need to deploy cables, which may be costly to maintain. Depending on the scenario or the particular control loop, the communications for factory automation could require very low latency and/or high reliability could be very high.        
High reliability is one of the most important aspects of URLLC use cases. As can be seen in 3GPP TR 22.862, not all use cases in this family require latencies as low as 1 millisecond, but instead what is most important is that the messages get reliably delivered within a given latency bound. Reliability in this sense can be defined such that the ratio of lost, erroneous or delayed messages (i.e., exceeding the given latency bound) should be very low. If, for example, transmission of messages is required to be provided at a reliability level of 1-10−5, within a delay bound of 1 millisecond, then only 10−5 of the transmissions may either fail, or lead to latencies exceeding the 1 ms bound. In other words, for a URLLC service, the successful in-time transmission of messages needs to be guaranteed for the service up to the given reliability level for the defined latency bound.
This is illustrated in FIG. 1, which shows an example cumulative distribution function (CDF) indicating the percentage of transmissions delivered within a given latency. As shown in the figure, at least (100-ε)% of the transmissions should be delivered within the latency bound, where ε defines a maximum failure level for the corresponding service, in percentage terms, and thus 100-ε defines the minimum reliability level.
Another characteristic that is important for some URLLC use cases is high availability of the service. High availability is related to the end-to-end availability of an adequate communication path between the applications of the communication endpoints. The availability is specific to a service (or a class of services). For example, the availability provided by a specific network deployment and configuration for low-rate services with required high reliability but modest latency may differ from the availability provided to high-rate services with modest reliability and very low latency. Furthermore, the availability of services needs to be considered within the area of interest for a certain service. For example, for industrial factory automation, availability may be relevant only within the factory area, whereas for remote control of vehicles a much larger geographic area is relevant concerning the availability.
The communication path between the communication end points will be made up of radio links as well as transport links and different hardware and software-implemented functions. These resources may be deployed using redundant components and links, thus providing high availability. The provision of high availability is very much up to the operator offering the URLLC service.
Low latency is another service characteristic frequently mentioned in relation to URLLC, in 3GPP TR 22.862. For future 5G cellular networks, target latencies for URLLC are 0.5 milliseconds for both uplink and downlink communications. It is important to note though that many URLLC use cases may not require such low latency, and/or may require low latency in only one direction. Thus, 3GPP TR 22.862 describes use cases ranging from those that require 1-millisecond end-to-end latencies to others that are satisfied with multi-second latencies. Many URLLC services are most interested in the latency bound, i.e., the largest message latency that is expected to be achieved with a high reliability, as discussed above and in connection with FIG. 1. Transmission with lower latency than the bounded latency may not provide any benefits for these services. Generally, it is expected that there will be a trade-off between reliability and latency, e.g., a system may be able to provide 0.5-millisecond latency with a relatively low reliability, while much higher reliability is achieved (e.g., thanks to re-transmissions) if the latency can be longer.
It is difficult for a cellular operator to provide high-reliability services in wireless networks due to such factors as mobility of the wireless devices (referred to as “user equipment” or “UEs,” in 3GPP documentation), varying signal coverage, congestion, variable system loading, interference. In an indoor controlled environment, such as a factory, it may be possible with traditional planning methods to provide a high-reliability service, but for wide-area coverage more statistically based methods are required. Most wireless network deployments are not controlled environments, in that they are affected by a number of external factors that are not readily controlled by either the wireless operator or the customer.